Volcanoes and volcanic lakes, such as Lake Averno, Bolsena and those around Rome were all sacred places for the Greeks, Etruscans and other early settlers. Ancient man called volcanoes omphalos, after the Greek word, umbilical. They believed that volcanoes provided an entrance to the underworld; a way to connect the heavens with the world of the afterlife below. Remnants of altars and votive offerings have been found in many volcanic areas. Interestingly, volcanoes have been sacred in places and cultures that have had no contact with one another. It seems that the mystique of a volcano fascinated man from the dawn of time.
But that was then and this is now and we know so much more.
In general, there are two types of volcanic eruptions: effusive and explosive.
Effusive eruptions are what most people think of when they think of a volcano: steaming, hot lava bubbling out and over the rim and oozing down the side. Historically Mt. Etna has produced effusive activity; however several Plinian eruptions have been identified eons ago. Mt. Etna has the longest period of documented eruptions in the world, 3,500 years.
While effusive eruptions are fairly rare here, there are a few areas in the Campi Flegrei in which the lava can be seen: beneath the Accademia Aeronautica in Pozzuoli, in the crater of the Astroni volcano, at Agnano at the base of Montespina, and at Cuma.
Explosive eruptions have magma which is not very fluid, and are characterized by the emission of ash, pumice, volcanic debris (like stones, dirt, dust, etc.) and a large quantity of gas. The deposits from these eruptions are called “pyroclastic” (fiery rock fragments). Volcanoes of this type are Vesuvius (sometimes), Stromboli and the volcanoes of the Andes to name a few. Vesuvius is very tricky and changes its eruptive style so more about that later.
Explosive eruptions can be further divided into two categories:
- Eruptions caused by the expansion of gases contained in the magma; these are referred to as magmatic eruptions.
- Eruptions produced by the vaporization of surface water (i.e. oceans, lakes, etc.) coming into contact with magma, these are called hydromagmatic.
When we think of volcanoes we have to think in terms of their releasing a buildup of gas found in the magma. The diagram below shows a cross section of a volcano. Think of it like a belly button with the umbilical cord reaching far into the center of the earth where molten magma resides. Depending on the physical and temperature forces, the gases contained in the magma can either trickle out slowly or burst forth with a devastating blast.
I just had to include this because it IS truly incredible footage of the Marum Volcano on Ambrym Island in the South Pacific nation of Vanuatu.
To wrap up this installment, here is something for all of you who are fans of David Letterman’s Top Ten Lists. Name the world’s top ten active volcanoes.
Next Up: The Burning Fields
Ann Pizzorusso is a geologist and Italian Renaissance scholar. After many years of doing virtually everything in the world of geology (drilling for oil, hunting for gems, cleaning up pollution in soil and groundwater) she turned her geologic skills toward Leonardo da Vinci. See her work on Leonardo’s Geology. You can find Ann on Facebook at Leonardo da Vinci Virgin of the Rocks and on Twitter @VirginoftheRock.